In U.S. Pat. No. 4,728,499 and the continuation thereof, the disclosure of which is incorporated herein by reference, there is disclosed a combination rapid response device for the detection of carbon dioxide in a concentration of more than 2% in a gas mixture which includes an indicating element, comprising a chromogenic pH-sensitive indicator and other substances, which indicating element provides a visual color change within 5 to 20 seconds when the concentration of carbon dioxide in the gas under investigation is 2% or more. In the preferred embodiment of the device said indicating element is impregnated on to a carrier to form an indicator component which is mounted within an enclosure having an inlet, an outlet and a transparent window through which the component and any color change thereof may be viewed.
Thus, according to U.S. Pat. No. 4,728,499, the nature and concentration of the substances in the indicating element give a rapid, positive color change when the concentration of carbon dioxide in the surrounding gas reaches or exceeds a minimum concentration of 2%, thereby providing a device which enables the positioning of an endotracheal tube in a patient to be determined accurately and rapidly.
However, there are some instances wherein it is necessary to determine more quantitatively the concentration of carbon dioxide in a gas, not merely a given minimum concentration. Also, it is advantageous to have a device and method which provides on-line, real time, repetitive measurement so that temporal changes in carbon dioxide concentrations may be detected.
It has now been found that these further objectives may be achieved by extending the visual detection concept utilized in the device of U.S. Pat. No. 4,728,499 to (i) a component capable of giving progressively different response signals or (ii) a sequential array of components, each of which is pre-calibrated to give a rapid response signal, for example a visual response (color change); each of said response signals in either embodiment corresponding to a different pre-determined concentration of carbon dioxide in the gas under investigation, said embodiment being capable of responding repetitively to changing concentrations of carbon dioxide.